Unset

lhs =. removes any rules defined for lhs.

Examples

Remove a definition:

x = 5;
x
(* 5 *)

x =.;
x
(* x *)

Remove function definition:

f[x_] := x^2;
f[3]
(* 9 *)

f[x_] =.;
f[3]
(* f[3] *)

Please visit the official Wolfram Language Reference for more details.